2017年九寨沟MS7.0地震震源区横波分裂变化特征

对2017年九寨沟M_S7.0地震序列的横波分裂的时空变化特征进行了分析.通过横波窗内S波质点运动图的分析，从九寨沟地震震源区各个地震台站的近震横波记录中提取了横波分裂的快波偏振方向和慢波延迟时间.观测结果显示，震源区各台站的上地壳各向异性在空间上存在分区特征，时间上有随时间的趋势性变化特征.空间上，位于震源区北部余震区内的3个台站中，发震断层东面的台站L5112和L5111只有一个突出的快波偏振优势方向（NNE向），而西面的L6202台有两个快波偏振优势方向（除了NNE向，还有一个近EW向），体现了余震区剧烈调整的地壳应力和构造复杂断裂的综合作用；余震区外的3个台站中，震源区东部靠近塔藏断裂（东）附近的JZG台的快波偏振优势方向为NW向，与塔藏断裂（东）的走向一致，南部的台站L5110和L5113的快波偏振优势方向为近EW向，与区域主压应力方向一致；余震区内各台站的平均慢波延迟时间大于余震区外各台站，反映了九寨沟地震孕育过程中余震区的应力积累强于其周边区域.时间上，快波偏振方向在主震后前期离散度较大，随着时间的推移，离散度在后期有逐渐变小的趋势；慢波延迟时间在主震后较大，但随着时间的推移，也表现出逐渐减小，趋于稳定.横波分裂随时间逐渐减小和趋于稳定的变化特征反映了九寨沟地震在孕震中积累的应力，随着主震和余震的发生而导致的应力释放和调整，应力大小和调整幅度逐渐减小，后期趋于稳定.

Variations of shear wave splitting in the source region of the 2017 Jiuzhaigou MS7.0 earthquake

This work studied the variation of shear wave splitting associated with the 2017 Jiuzhaigou M_S7.0 earthquake sequence. By analyzing shear wave particle motion of local events in the shear wave window, the fast polarization directions and the delay times between fast and slow shear waves were derived from seismic recordings at seven stations in the source area of the Jiuzhaigou earthquake. In the study region, the fast polarization directions show partition characteristics. And the systematic changes of the delay times between two split shear waves were also observed. As for spatial distribution, there are three stations L5111, L5112 and L6202 in the aftershock area of the northern source area of the Jiuzhaigou earthquake, of which the former two located on the east side of the seismogenic fault of the Jiuzhaigou earthquake have only one predominant fast direction (NNE), while the latter one located on the west side of the seismogenic fault has two predominant fast directions (EW and NNE), implying a combining effect of the drastic change of crustal stress and complex fault structure in the aftershock area. With reference to the three stations JZG, L5110 and L5113 outside the aftershock area, the NW fast polarization direction of station JZG located in the eastern source area and near the Tazang fault (east) is consistent with the strike of the Tazang fault (east), while the EW fast polarization directions of stations L5110 and L5113 in the southern source area are consistent with the direction of regional principal compressive stress. The average delay times of the stations in the aftershock area are greater than those of the stations outside the aftershock area, indicating that the stress accumulation in the aftershock area is stronger than that of the surrounding area during the seismogenic process of the Jiuzhaigou earthquake. In the time domain, the dispersion of the fast directions was large in the initial period after the main shock; as time went on, the dispersion became smaller; the delay times was also large in the initial period after the main shock, and gradually decreased with time and stabilized after a period of time. The variations of shear wave splitting with time may demonstrate changes of the local stress field during the main shock and the aftershocks. With the stress release and adjustment caused by the main shock and aftershocks, the stress accumulated during the seismogenic process of the Jiuzhaigou earthquake gradually decreased, and the stress tended to stabilize in the late stage.